In this lesson, you will learn how to use kernel-level tracing to witness exactly how the Virtual File System (VFS) evaluates complex ACLs during a file access request.
When standard commands like getfacl fail to reveal why a user is denied access—perhaps due to complex mask interactions or inheritance conflicts—we need to look at what the kernel actually does. The VFS layer is the gatekeeper that mediates every file operation; by tapping into its functions, we can see the logic gate-by-gate as the kernel calculates whether to grant or deny access.
Tracing the vfs_permission Logic
The kernel uses the vfs_permission (or inode_permission in newer kernels) function to evaluate if an operation is allowed. By using ftrace or bpftrace, we can inspect the decision-making process in real-time.
For example, you can use a bpftrace one-liner to watch the kernel check permissions for a specific file:
sudo bpftrace -e 'kprobe:generic_permission {
printf("Checking permissions for UID %d on inode %llu\n", uid, arg1);
}'
When you trigger a file access, the kernel traverses the inode’s security structures. If an ACL is present, the kernel invokes posix_acl_permission to resolve the effective rights. By tracing this specific function, you can observe whether the kernel is hitting the "mask" entry or correctly identifying a "named user" entry.
Why this is powerful
Traditional tools show you the state of the ACL, but they don't show you the intent of the kernel during a specific system call. VFS tracing allows you to see:
- Mask Overrides: Confirm if the
maskentry is silently stripping permissions you thought were active. - Evaluation Order: Verify if the kernel stops at the first matching entry or continues through the entire ACL chain.
- Context Mismatches: Detect if the process’s Effective UID differs from what the VFS layer is actually evaluating, which often happens with SUID binaries or complex container namespaces.
By shifting your view from the static filesystem metadata to the dynamic kernel execution path, you move from guessing about permissions to witnessing the security policy in action.
Key Takeaway: VFS tracing allows you to bypass the static view of ACLs and observe the kernel’s real-time logic as it resolves complex, layered permission chains during file access.